There are several ways to measure the ring size at home.
If you are shopping for your self. The internal diameter measures the inside part of another ring. Where as, the circumference can be measured by using a string or paper method around the finger the ring is to be worn. 

Internal Diameter	Circumference	UK	US	EU	AU
14.5mm	45.5 mm	G	3.75	45	6
14.8mm	46.5mm	H	4.25	46.5	7
15.2mm	47.8mm	I	4.5	47.5	8
15.6mm	49.0mm	J	5	48.5	9
16.0mm	50.2mm	K	5.5	50	10
16.4mm	51.5mm	L	6	55	11
16.8mm	52.8mm	M	6.5	52.5	12
17.2mm	54.0mm	N	6.75	53.5	13
17.4mm	55.0mm	O	7	55	14
18.0 mm	56.5mm	P	7.5	56	15
18.4mm	57.8mm	Q	8	57.5	16
19.0mm	59.0mm	R	8.5	59	17
19.2mm	60.5mm	S	9	60.25	18
19.5mm	61.5mm	T	9.5	62.75	19
20.0mm	62.8mm	U	10	63	20
20.3mm	64.0mm	V	10.5	64	21
20.7mm	65.3mm	W	11	65	22
21.1mm	66.6mm	X	11.5	66.5	23
21.4mm	67.5mm	Y	12	67.5	24
21.8mm	68.7mm	Z	12.5	69	25

 

When shopping for someone else, the best way to find their ring size is to ask them directly. If you're planning a surprise, get help from friends and family. They can ask without raising any eyebrows.